This invention is related to the sensing of the descent of oil well tools in an oil well bore, and is more particularly directed to the indication of the arrival of an oil well tool such as a cementing plug at a predetermined position in the well bore during operations in the oil well such as a cementing operation.
In an oil well cementing operation, generally two plugs are used to separate the cement from other fluids in the oil well. Generally, a first plug is released and cement pumped above the plug for separating the cement from fluid in the well. After the proper amount of cement has been pumped into the well, a second plug is released for separating the cement from fluid above the cement. Additional fluid is then pumped into the well to displace the cement through the well casing. When the first plug reaches the bottom of the well, it drops out of the center string and the cement following the first plug is displaced out the bottom of the center string and up the annulus surrounding the string until the well annulus is completely filled with the cement. In the desired operation the volume of the cement is closely calculated such that when the well annulus is filled, the second plug has reached the bottom of the string and wiped the interior of the string clean of cement during its descent.
In offshore operations, the cement used in the operation is delivered to the casing string to be cemented into place by a smaller diameter drill string. In this case, a larger second plug is held in place by a specially designed latching mechanism at the lower end of the smaller drill string. A specially designed top releasing plug is released from the surface and is pumped down behind the cement. The cement flows through a center bore in the second plug into the casing and from there into the well in the conventional manner. The top releasing plug seats in the second plug, and the second plug is then released and the two plugs in sealing relationship are then pumped behind the cement in the conventional manner.
In the cementing operation, it is advantageous to know when the top releasing plug has reached the second plug in order to determine, among other things, when the full amount of cement has been pumped out of the center of the casing string and into the annulus surrounding the string.
In the past, the pressure of the fluid pumping the top releasing plug into the borehole has been observed and when the top releasing plug seats in the second plug an increase in pressure is observed until the second plug is released. However, many times the second plug is released without the mentioned pressure increase.
In the Smith et al U.S. Pat. No. 2,999,557 issued Sept. 12, 1961, a transmitter is located in a housing above the borehole to send acoustic pulses down the borehole. The pulses are reflected by a plug which is made of a substance having a high acoustic impedance. The reflected pulses are then detected by a microphone and processed and displayed on a recorder to provide an indication of the location of the plug as the plug descends into the borehole.
In the present invention, an acoustic transmitter is positioned at a desired location in the drill string in the borehole. The transmitter gives an acoustic signal when it senses the passing of the well tool to give a surface indication that the well tool has reached the predetermined location of the transmitter. Further, the use of a surface indicating means is disclosed which includes a visual indicating means and an acoustic indicating means responsive to the signals received from the down hole transmitter.
In the preferred embodiment disclosed, a magnet is attached to the plug to be sensed. The mentioned magnet causes a magnetic switch in the acoustic transmitter to close. A circuit is disclosed wherein the magnetic switch controls the acoustic signal. The disclosed embodiment has an oscillator which drives the acoustic transmitter at one frequency and whose frequency of oscillation is changed upon the momentary closing of the magnetic switch.